Composite films having a plurality of films adhesively bonded with a laminating adhesive are widely used in the field of packaging materials.
Two-part laminating adhesives composed of a base composition containing polyol and a curing agent containing polyisocyanate have been widely known hitherto.
In recent years, there has been studied the development of a two-part solventless laminating adhesive without containing any organic solvent from the viewpoints of reduction of environmental load and improvement of working environment.
For example, there has been proposed a solvent-free two-part curable adhesive composition containing a polyol component (A) and a polyisocyanate component (B), the composition having a viscosity of 900 mPa·s or more at 80° C. immediately after the components (A) and (B) are mixed together (see, for example, the following Patent Document 1).
Also, there has been proposed a solvent-free two-component curable adhesive composition containing a polyol component (A) and a polyisocyanate component (B), in which at least either component (A) or (B) contains at least one polyol component selected from the group consisting of a polyether polyol, a polycarbonate polyol, and a polyurethane polyol, and exhibiting crystallinity at room temperature; and a proportion of the crystalline polyol component is 3 to 50% by weight of the total weight of the components (A) and (B) (see, for example, the following Patent Document 2). The solvent-free two-component curable adhesive composition has a viscosity of 100 to 1500 mPa·s at 70° C. immediately after the components (A) and (B) are mixed together, and an increasing ratio of the viscosity after the composition is stood at 70° C. for 10 minutes is 120% or less relative to the viscosity immediately after the mixing.
Further, there has been proposed a two-part curable solventless adhesive, in which a base composition contains a polyol having an acid group and a secondary or tertiary terminal hydroxyl groups accounting for 30% or more of the entire terminal hydroxyl groups, and a curing agent contains an isocyanate group-terminated urethane prepolymer having an average functionality of 1.5 to 2.5 which is obtained by reaction of an aralkyl polyisocyanate and a polyol, and an aliphatic polyisocyanate, and/or a modified substance thereof (see, for example, the following Patent Document 3).
The two-part solventless laminating adhesive described above is obtained by mixing the base composition and the curing agent in a solventless laminating apparatus, and the mixture is coated onto a film which passes through between a pair of rolls each rotating in the same direction in opposed relation to each other. Subsequently, the film is adhesively bonded to the other film with a nip roller, to thereby produce a composite film.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-172602    Patent Document 2: Japanese Unexamined Patent Publication No. 2002-249745    Patent Document 3: Japanese Unexamined Patent Publication No. 2006-213801